about database and transactions...


A database is in a consistent state X.
If something wants to modify the database such that it will be in another state Y,
then a transaction T will be linked with the last DB state (currently X).
Linked in such a way that will tell the system the transaction is open on state X.
All modifications will be done within this transaction such that any reads will 
pass thru T then thru X and any writes will only be stored in T only.
When T is commited, X+T may get to state Y, but state X won't be destroyed 
if it has links to other transactions. Links that imply open transactions.
If all transactions are done on state X then X may be disposed of. And only the
next state Y can be kept , whereas Y is the result of X+all transactions started on X.


A transaction TT within a transaction T implies that from the point of view of TT
X+T becomes a DB state Y such that, TT is started on Y. And so on of deeper 
transaction within transaction levels.
A Sibling transaction has access to the above sibling transaction in the sense that
it sees all modifications done by it.

AtKaaZ, 15 Dec 2008.